LaNi5 alloys have been extensively studied for use in hydrogen storage and Ni—MH batteries. This is because of their ability to absorb and desorb large quantities of hydrogen (up to 6.7 hydrogen atoms per formula unit). Further, these alloys have moderate equilibrium plateau pressures at room temperature. However, the reversible storage capacity of pure LaNi5 degrades unacceptably during thermal cycling. To overcome this problem substitutions for both La and Ni have been employed. Even with these substitutions, there exists another problem, particularly with respect to the electrochemical performance of the pure and substituted LaNi5 alloys.
The problem relates to the discharge of Ni-MH batteries using the LaNi5 type alloys. The problem is that as the battery is discharged, the voltage of the cell falls off long before the charge of the cell is depleted. This voltage drop is undesirable and can cause issues with devices employing the battery. It is preferable for the voltage of the cell to remain at the proper level for as much of the discharge cycle as possible. This effect can be correlated to the shape of the PCT curve for the hydrogen storage alloy. The typical PCT curve includes three “regions”. The first is an initial high slope region where a small increase in the stored hydrogen increases the equilibrium pressure rather quickly. The second region is known as the plateau pressure region. In the plateau pressure region, the concentration of stored hydrogen can increase significantly with only a small or moderate increase in equilibrium pressure. The third region is another high slope region where once again small increases in concentration result in rapid increases in the equilibrium pressure.
The discharge voltage problem discussed above is related to the slope of the first region on the PCT curve. The steeper the initial slope of the PCT curve, the longer the cell will hold its working voltage during the discharge cycle. Conversely, the shallower the slope of the first region, the earlier in the discharge cycle the voltage begins to drop. Unfortunately, the LaNi5 type alloys have relatively shallow slopes in the first region of the PCT curve.
NdNi5 alloys are known to have storage capacity similar to that of LaNi5 alloys. Further, the inventors note that the slope of the first region of the PCT curve is very steep for the NdNi5 alloys, thus affording Ni-MH batteries made with these alloys with the ability to hold their cell voltage much longer into the discharge cycle than their LaNi5 alloy counterparts.
Unfortunately, NdNi5 alloys suffer from equilibrium plateau pressures at least an order of magnitude higher than those of the LaNi5 alloys. This extremely high plateau pressure is unacceptable for use in Ni-MH batteries. This very high plateau pressure causes the battery to require a containment vessel that can handle such high pressures, and ultimately causes the cell to have a low storage capacity (because only a fraction of the total capacity is available at reasonable containment pressures.
Thus, there is a need in the art for a NdNi5 type alloy which has both the steep slope of the first region of the PCT (for good discharge voltage characteristics) and a reasonable plateau pressure (for reasonable room temperature storage capacity).